Fix control_flow_ops (tflearn#380)

Tensorflow 0.11 removes the ability to access python.control_flow_ops
tf.cond is (and was) the correct function
REF: tensorflow/tensorflow#4616

tflearn/layers/normalization.py

@@ -88,7 +88,7 @@ def update_mean_var():
 
         # Retrieve variable managing training mode
         is_training = tflearn.get_training_mode()
-        mean, var = tf.python.control_flow_ops.cond(
+        mean, var = tf.cond(
             is_training, update_mean_var, lambda: (moving_mean, moving_variance))
 
         try: